Cancer (malignant tumor) is a disease ranked as the first leading cause of death, for which new therapies are needed. At present, therapies for malignant tumors include surgical therapies, radiotherapies, and chemotherapies (anti-malignant tumor agents), and these therapies are usually combined for treatment. Anticancer agents used include alkylating agents, antimetabolites, alkaloid anticancer agents, anticancer antibiotics, platinum-based drugs, molecular targeted drugs, etc. These anticancer agents still cannot be considered to have a sufficient therapeutic effect and also have the problem of high frequency of side effects.
On the other hand, indirubins, which are indole compounds isolated from human urine, are known to have higher affinity for the aryl hydrocarbon receptor (AhR) than 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an environmental hormone. Thus, it is suggested that indirubins may be an endogenous ligand for the AhR (Non-Patent Literature 1). It is also reported that indirubins act on cyclin-dependent kinases (CDKs) and glycogen synthase kinase-3β (GSK-3β) to influence cell cycle, cell differentiation, nerve cell polarization, and so on (Non-Patent Literature 2). These target proteins for indirubins are highly involved in cell proliferation, and GSK-3β is a tyrosine kinase which is a target of recent molecular targeted drugs for cancer therapy.